Mixed salts of tripolyphosphates (sodium and potassium) have been found to be highly desirable for use in food preparation and preservation because of the properties of such salts. While sodium tripolyphosphates have been employed as a detergent builder and in food products for many years, it was known that potassium tripolyphosphate was much more soluble in water and dissolved at a faster rate than sodium tripolyphosphate. Furthermore, its value in food products has been appreciated when the amount of sodium in the human diet was brought to the attention of the public by the medical profession. Of course, the use of the potassium salt would reduce the amount of sodium in food products and it is desireable to have a detergent builder that is readily soluble in water. However, there are many difficulties associated with the handling and use of potassium tripolyphosphate because it is deliquescent, prone to severe caking and difficult in general to ship, store and handle conveniently. Also, the potassium salt of tripolyphosphate is widely known to impart an undesirable taste to food products and therefore has not been used for that purpose.
One solution to the problems presented by the properties of potassium tripolyphosphate is the use of mixed sodium/potassium salts of tripolyphosphate. Such use would reduce the amount of sodium in the diet and the mixed salts exhibit increased solubility in water over the sodium tripolyphosphate salts. Cost has been one major factor which has limited the use of the mixed salt. More importantly, there has not been found a convenient, efficient process for producing large quantities of the mixed salt.
One early attempt to produce the mixed salts of tripolyphosphate is disclosed in U. K. patent 1,102,892. Such process starts with the phosphate ore which is digested with a solution of phosphoric acid. Sodium and potassium sulfate salts are then added to the solution which causes the calcium content of the ore to precipitate, leaving a solution of mixed orthophosphate salts. After filtering the solution containing the mixed salts is solidified in a multi-action evaporator. First, the water is removed from the solution to a sufficient extent to either eliminate free water or provide a concentrate containing between 48% to about 55%, by weight solids. If only a concentrated solution is left, it is mixed with an amount of desired end product, i.e., sodium potassium tripolyphosphate, previously prepared. In either case the dried product is then calcined at the usual temperatures employed to produce the mixed salt tripolyphosphate.
Another method of preparing the mixed tripolyphosphate salt is described in U.S. Pat. No. 3,372,984 to Metcalf et al. In this process a very concentrated aqueous slurry of sodium tripolyphosphate is prepared to which are added the appropriate amount of sodium and potassium bases. It is essential that the polyphosphate reactants are largely in the undissolved state. The dihydrate of the mixed salt is produced.
The improved solubility of the mixed tripolyphosphate salt is appreciated in the prior art as is shown by the disclosure of U.S. patent 4,857,287 to Dick et al. The product of the process disclosed therein desirably contains more that 75%, by weight, Form I sodium tripolyphosphate and 0.4% to about 0.8% potassium. According to said process an aqueous solution of sodium orthophosphate wherein the Na.sub.2 O/P.sub.2 O.sub.5 ratio is from 1.60 to 1.67 and an appropriate amount of dissolved potassium is sprayed into a spray dryer maintained at a calcining temperature so as to produce tripolyphosphate. The product is reported to have improved caking properties and quickly dissolves in water.
Other mixed salts of condensed phosphates have been prepared by various processes. In U.S. Pat. No. 2,511,249 to During there is disclosed a process for preparing sodium potassium pyrophosphates. In this process a solution of mixed sodium and potassium orthophosphates is dried on a drum drier or other suitable surface and then subjected to calcining to form the trimetaphosphate which is reported to be water soluble.
An improvement in the process for preparing the sodium salt or sodium/potassium double salt is reported in U.S. Pat. No. 3,230,040 to Metcalf et al. Avoidance of water insoluble metaphosphates is achieved by rapidly increasing the temperature of the dried mixed orthophosphate salts. A solution of mixed orthophosphate salts is first dried on a drum drier and then prepared for calcining by appropriate milling and screening. Rapid heating such as by gas fired jets directed at the surface of the dried salt mixture achieves the objective of avoiding insoluble metaphosphates.
Preparation of other mixed salts, i.e., calcium/sodium of metaphosphate is described in U.S. Pat. No. 4,725,369 to Nathan et al. A slurry of mixed salts is heated to remove water and then calcined to provide the desired mixed salt metaphosphate in substantially crystalline form.